Authentication Protocols

Authentication Protocols Celia Li Computer Science and Engineering York University

Outline • Overview of authentication • Symmetric key authentication protocols (Kerberos) • Public key authentication protocols (SSL) • Authentication protocols in Wireless Local Area Networks (WLAN)

Outline • Overview of authentication • Symmetric key authentication protocols (Kerberos) • Public key authentication protocols (SSL) • Authentication protocols in Wireless Local Area Networks (WLAN).

Overview of Authentication • The process of determining whether someone is, • in fact, who it is declared to be. • An authentication protocol consists of a sequence • of messages between principals and will be • described using different notations. • Principals: parties who cooperate by exchanging • messages over networks.

Notation • E(K : M): denotes encrypting message plaintext M • with key K. • Ka: key of principal A • Kab: shared key of principle A and B • Principals are generally denoted by capitals such as • A, B and S (server) • (1) A B : M1 • (2) B S : M2 • (3) S B : M3

Notation • Na:A number generated by a principal A. Generally • termed as a nonce. • A nonce could be • a timestamp: a number denoting the current time. • a sequence number • a random number • A B : A, E(Kab : Na)

Overview of Authentication • Timeliness in Authentication Protocols • An authentication protocol is required to guarantee that the parties involved in the authentication process are present during the execution of the protocol. • Nonces are generally used to assess timeliness in authentication protocols.

Attacks on Authentication Protocol • Replay attack: • The attacker records the message of an authentication protocol and replays this information to attempt to falsely authenticate to the other principle. • Counter measures: • Using timestamps, sequence number or random numbers.

Symmetric Key Authentication Without Trusted Third Party • Symmetric key one-pass unilateral authentication protocol B authenticates A: If Na (timestamp) has a recent value, B can prove A’s identity. • Symmetric key two-pass unilateral authentication protocol B authenticates A: • Symmetric key three-pass mutual authentication B authenticates A: A authenticates B: Na: Timestampt , Ra, Rb: Random numbers.

Symmetric Key Authentication With Trusted Third Party • NS (Needham-Schroeder) Symmetric Key Authentication A & B trust S, Kas is a shared key of A & S, Kbs is a shared key of B & S • A gets a shared key Kab generated by S • B gets shared key Kab • A authenticates B • B authenticates A S: Trusted Third Party (1) (2) (3), (5) A B (4)

Kerberos Authentication • A typical example of symmetric key • authentication with trusted third party • The trusted third party in Kerberos is the • authentication server which is in charge of • distribute shared keys. • Kerberos ticket is employed for the • authentication purpose

Kerberos Authentication • Kerberos Ticket • A certificate issued by an authentication server, encrypted using the key of the authentication server. • The ticket contains • the key that will be used for authentication of the client to the verifier who will verify the correctness of the key • the name of the principal to whom the key was issued • an expiration time of the ticket

Kerberos authentication • Client C wants to access server S. Client C and sever S need to authenticate • each other • Authentication server is comprised of two parts: • A: Key Distribution Server • G: Ticket Grant Server • (1) C sends a random number N1 to A • (2) A sends back a ticket Tcg to C. • Kac: shared key between A and C • Kcg: shared key between C and G • C authenticates A if N1 is correct

Kerberos authentication • (3) C forwards the ticket Tcg to G • N2: random number • T: time stamp • G authenticates C if time stamp is correct • (4) G sends back a service ticket Tcs to C. • Kcs: shared key between C and S • C authenticates G if N2 is correct • (5) C sends ticket Tcs to S • T’: time stamp • S authenticates C if time stamp is correct • (6) C authenticates S if time stamp is correct

Public Key Certificate • Commonly used for public key authentication protocols • An electronic document which uses a digital signature to bind together a public key with an identity: • the name of a person or an organization, their address • Issued by a certification authority (CA) • e.g. VeriSign • The certificate can be used to verify if a public key belongs to an individual.

Public Key Certificate • Public key certificates include: • Public key of the user • DN (name of the user) • Validity period of the certificate • Name of the certificate issuer • Digital signature of the • certificate issuer

Public Key Authentication without Trusted Third Party Public key one-pass unilateral authentication protocol B authenticates A: Public key two-pass unilateral authentication protocol B authenticates A: Public key three-Pass mutual authentication protocol B authenticates A: A authenticates B: CertA: public key certificate of A Ka-1: private key of A, Kb-1: private key of b

Public Key Authentication with Trusted Third Party NS (Needham-Schroeder) Public Key Authentication Protocol A asks B’s public key from S S sends B’s public key Kb to A B gets A’s identity B asks A’s public key from S S sends A’s public key Ka to B A authenticates B by verifying Na B authenticates A by verifying Nb S: Trusted Third Party (5) (1) (4) (2) (3) (7) A B (6)

Secure Socket Layer Protocol (SSL) • Universally accepted on the Web for authenticated and encrypted communication between clients and servers. • Typical example of public key authentication • Protect sensitive information such as Social Security Number (SSN), Credit Card Number, etc. Web Server Client

SSL • Server Certificate • The SSL protocol requires a server certificate so that client can authenticate the server. • Client Certificate (Optional) • You can optionally configure your server to request a client certificate so that server can authenticate the client. • Only discuss how client authenticates server in SSL

SSL Client authenticates server based on certificate issued by certificate authority (CA) • Client  server: Hello message • Server  client: server’s public key certificate, including server’s public key, digital signature signed by certificate authority (CA) • Client uses CA’s public key to verify digital signature signed by CA, thus verifies server’s certificate. • Client  server: a random number • in order to prove server’s identity. • 5. Server encrypts random number with its private key. • 6. Server  client: the encrypted message • Clients decrypts the message using server’s public key to verify identity of the server.

Compare SSL and Kerberos

Authentication Protocols in WLAN • WPA (Wi-Fi Protected Access) • Certification program created to secure wireless computer networks. • Provides stronger data encryption and user authentication • Implements the IEEE 802.11i standard. • The three entities of WPA are the client, the Authentication Server (AS), and the Access Point (AP). AS • Only after the client is authenticated by the authentication server, the authenticator (Access Point) allows the client to access the network

Authentication Protocols in WLAN • Symmetric key approach: EAP-LEAP • Public key approach: EAP-TLS • Symmetric Key Approaches • Efficient and require little computational power • Resource constrain, e.g., computational power of PDAs, mobile VoIP phones, have. • Drawbacks • Most protocols derive the shared secret from the user's password and some passwords are not strong, it is easy for the attacker to extract the secret key from them.

Lightweight Extensible Authentication Protocol (LEAP) The server knows the password of the client. Password is the shared key between the client C and authentication server S

Authentication Protocols in WLAN • Public key Approaches • Provides strong security • Cannot authenticate a client without • public certificate • Example: EAP-TLS

EAP-Transport Layer Security (EAP-TLS) • AS verifies client’s digital signature using client’s public key got from client’s certificate Certclient • Get random number p by decrypting with its private key • Client calculates H(c,s,p), compares it with the value sent by As

References [1] Kerberos http://web.mit.edu/kerberos/www/ [2] SSL http://docs.sun.com/source/816-6156-10/contents.htm [3] EAP-TLS http://en.wikipedia.org/wiki/Extensible_Authentication_Protocol

Authentication Protocols